The present invention relates to improvements in a vertical roller mill. More particularly, this invention relates to a roller mill having a high pulverizing efficiency due to optimized mating of pulverizing surfaces of the roller mill.
In a roller mill, hard raw material, such as coal, is fed between pulverizing rollers and a substantially horizontal upper surface of a pulverizing table, which rotates around a vertical axis. The raw material is pulverized by compression between the pulverizing table and the rotatable pulverizing rollers. In one type of conventional roller mills, shown, e.g., in U.S. Pat. No. 4,234,132, the cross-sectional shapes of the pulverizing surfaces, i.e., the upper surface of the pulverizing table and the peripheral surface of the rollers, are flat. In another type of conventional roller mills, shown, e.g., in U.S. Pat. No. 3,366,338, the peripheral cross-sectional shape of the rollers is arcuate, and an annular groove having an approximately similar arcuate cross-sectional profile is provided in the pulverizing table.
When raw material is pulverized in a gap formed between the pulverizing table and the rotatable rollers, the shape of the individual pulverizing components affects the pulverizing efficiency and the general performance characteristics, such as the level of vibrations or power consumption, of the mill. Therefore, a variety of different modifications of the above-mentioned cross-sectional profiles of the pulverizing surfaces has been suggested.
U.S. Pat. No. 4,606,506 discloses a roller mill with arcuate pulverizing surfaces arranged radially on the outer side of the system and flat pulverizing surface cross sections, i.e., conical surfaces, on the inner side thereof. The purpose of this construction is to induce uniform wear of the rollers by avoiding high wear of the coarse grinding, i.e., the inner side of the system. Japanese Patent Publication No. 2026647A2 discloses a roller type crushing apparatus which provides improved crushing efficiency by arranging the crushing surfaces of the rollers and of the rotating table to have almost similar contours consisting of two circular arcs with different radii of curvature.
U.S. Pat. No. 4,611,765 shows a roller mill with at least one annular recess on the pulverizing surface of the pulverizing rollers. The system is also characterized by the gap between the pulverizing roller and the pulverizing table having a wedge-like sectional shape tapering towards the outer side of the pulverizing table. The narrow gap prevents the material from escaping too early from the pulverizing area.
Due to non-uniform wear of the pulverizing surfaces, their outer portions may protrude relative to the areas located closer to the center of the pulverizing table. U.S. Pat. No. 5,518,192 suggests a roller mill in which the outer portion of the pulverizing surface in a pulverizing roller is chamfered in order to avoid the formation of protruding areas and to ensure smooth discharge of the pulverized product to the outside of the pulverizing table.
The above-described modifications of the pulverizing surface profiles generally eliminate some of the deficiencies in most conventional roller mills, but the need for simple pulverizing surfaces with improved overall performance of the system still exists.
The object of the present invention is to provide a roller mill having simple pulverizing surfaces and which provides high performance.
More particularly, the object of the present invention is to provide a roller mill having pulverizing surfaces at a rotating pulverizing table and at rotatable rollers ensuring efficient pulverization.
Another object of the present invention is to provide a roller mill having pulverizing surfaces at a rotating pulverizing table and at rotatable rollers ensuring economical and reliable operation of the roller mill.
In order to achieve these and other objects of the present invention, a roller mill is provided, comprising: a base; a pulverizing table having a substantially horizontal upper surface supported against the base and arranged to be rotated around a vertical axis intersecting the upper surface of the pulverizing table in a center point thereof; an annular groove formed in the upper surface of the pulverizing table, adjacent to an outer periphery of the table; at least one roller shaft mounted pivotally on the base in proximity to the table at an angle xcex1 with respect to a horizontal direction toward the center point and having an end portion located above the table; a pulverizing roller supported rotatably against the end portion of the at least one roller shaft; and means for pressing the pulverizing roller toward the annular groove, wherein raw material introduced to the rotatable pulverizing table is pulverized by compression between the annular groove and the at least one pulverizing roller, and wherein an outer peripheral surface of the pulverizing roller has a smooth, generally arcuate cross section with at least one substantially flat section in the central portion thereof.
It is generally known that conventional concentric circular pulverizing surface profiles are subject to non-uniform wear of their surfaces. The present invention is based on the surprising observation that the above-mentioned features of pulverizing surfaces are especially advantageous in a pulverizing surface profile. The general idea of the present roller mill design is to arrange a large portion of the grinding surfaces in contact with each other under a typically applied load. This is achieved by well-matching the pulverizing surface profiles as much as possible. Additionally, it has been found that certain, apparently minor, details of the pulverizing surfaces can be optimized to improve the efficiency and durability of the roller mill.
I have discovered that the width of the substantially flat section in the central portion of the peripheral surface of the roller should make up a suitable portion of the peripheral surface. The width of the flat section is preferably between about 20% and about 60%, more preferably between about 25% and about 40%, of the axial thickness of the roller, mainly depending on the characteristics of the material to be pulverized. By way of example, when coal is pulverized, the width being 30% of the axial thickness of the roller has proved to be suitable.
The flat section is to be located near the central plane of the roller, i.e., near the plane perpendicular to the roller axis at an equal distance from its inner and outer axial faces. However, according to a preferred embodiment of the present invention, the flat section is located closer to the inner axial face of the roller, i.e., the face which is toward the center of the pulverizing table, than to the outer axial face. Thus, according to the preferred embodiment, the substantially flat section extends for a distance W1 from the central plane of the roller toward the center of the pulverizing plate and for a distance W2 therefrom toward the outer edge of the pulverizing plate, whereby W1 is larger than W2.
A flat section in a generally circular contour forms a chord of the circle, and shifting this chord of constant length results necessarily in changing its direction. Correspondingly, according to a preferred embodiment of the present invention, the flat section is not parallel to the axis of the roller, but somewhat more horizontal. Usually, the axis of the roller is slanted at an angle xcex1, typically about 15xc2x0, toward the center of the pulverizing table. The direction of the flat section is preferably closer to the horizontal direction than the axis of the roller. Preferably, the flat section forms an angle xcex2 with the direction of the roller axis, xcex2 being between 0xc2x0 and xcex1, preferably between about 2xc2x0 and about 6xc2x0, most preferably about 4xc2x0.
The advantageousness of the above-described preferred direction of the flat section is based on the resulting optimized shape of the gap formed between the pulverizing surfaces. The radial flow of pulverized material slows down in the widened central portion of the gap, and thus, there is enough time for thorough grinding of the material. No noticeable recess is, however, formed in the central part of the peripheral surface of the roller, and thus, the load is distributed evenly over the entire surface thus, contributing to the grinding. Moreover, due to the smoothness of the pulverizing surfaces, the movement of the material is stable, and the surfaces are not prone to irregular wear or breakage. A correct angle of the flat section is also important for providing a stable flow of material through the gap.
The annular groove in the outer edge portion of the top surface of the pulverizing table has a generally arcuate cross-sectional profile. The profiles of the groove and roller define the shape of the gap between the pulverizing surfaces. According to a preferred embodiment of the present invention, the cross-sectional profile of the groove has a first radius of curvature in the radially inner portion of the groove and a second radius of curvature in the radially outer portion of the groove, the first radius of curvature being smaller than the second radius of curvature.
The groove with the above-mentioned profile and the roller profile provided with a flat section, together define a gap which has a minimum height in the radially inner portion thereof and another, local minimum height in the radially outer portion thereof. This shape has proved to be especially advantageous for efficient grinding and low and uniform wearing of the surfaces. The outer minimum height of the gap is preferably only slightly smaller than its maximum height in the central portion of the gap. The narrowest point at which the height of the gap is typically at most about 50% of its maximum is preferably in the inner portion.
Due to the contour of the present roller, it will ride over the material in the annular groove rather than being forced to plow through the total depth of material as do similar radius crushing surfaces. The energy consumed by the plowing effect is lost and not directed to actual crushing. The new design reduces this energy loss and allows material to enter the crushing zone with less resistance. This consumes less of the available rotating table power and is, therefore, more efficient.